This invention relates to an electronic circuit for simulating the resistance characteristics of a variable resistance sensing element, and more particularly to such a circuit that varies the value of an apparent resistance at its output in proportion to the value of a voltage applied at an input to the circuit.
In an actual implementation of a closed-loop system that controls a variable based on a feedback parameter such as temperature, a temperature-sensitive transducer element, such as a thermistor or a platinum probe, is utilized to generate the signal indicative of the actual value of the parameter. The resistance or impedance of the transducer varies in some known proportion across the temperature range of interest. This resistance is then used as the feedback parameter and is compared to a desired resistance value. Any difference therebetween is typically used to drive an actuator to adjust the overall system to match the actual and desired resistance values, and thus the temperature values.
For example, an aircraft environmental control system ("ECS") controls, among other things, the temperature of the air in the aircraft cabin and cockpit. The temperature at various locations in the cabin is sensed by temperature-sensitive probes, such as thermistors or platinum probes, and signals indicative of the various temperatures are provided as resistance values to a central processor. The processor runs various software routines, one of which compares the sensed temperature with a desired temperature value for each of the various portions of the cabin. If there exists any difference in the actual and desired temperature values for any region of the cabin, then the central processor commands an actuator, which moves an air inlet valve to adjust the temperature in that region of the cabin.
During the laboratory development of such an aircraft ECS, or the laboratory testing of an existing ECS, it is often required to emulate the sometimes complex resistance characteristics of the temperature sensing element on a test rig or fixture. A temperature sensing element, such as a thermistor or platinum probe, may have a positive or negative temperature coefficient. One known emulation technique is to use a variable resistance device, such as a potentiometer, to simulate the thermistor or platinum probe across its entire operating range. However, the potentiometer is a manually-adjustable device. As used in such a setup, a person must measure the desired commanded value of the temperature, usually expressed as a voltage, and then must use extreme caution to adjust the potentiometer as best as possible to match the commanded temperature value. This is an inaccurate technique in that the value of the potentiometer is determined by reading the number of turns off of the potentiometer dial and then calculating the resulting value of the potentiometer resistance therefrom. This suffers in both speed and accuracy.
Other known, but inaccurate, techniques for simulating a temperature-dependent resistance includes a resistance or impedance bridge circuit, and a switch decade resistance box. A bridge often becomes unstable over time. Also, the bridge requires a manual adjustment through use of a potentiometer, to provide a desired resistance value. On the other hand, the switch decade box is a cumbersome tool that utilizes a plurality of different resistor values. Thus, the decade box cannot provide the continuous "analog-type" resistance variance that a potentiometer can.